A Search for Variable Stars in the Globular Cluster M4 with K2

Joshua J Wallace,Waqas Bhatti,Gáspár Á Bakos,Joel D Hartman

doi:10.3847/1538-4365/ab3849

Joshua J Wallace, Waqas Bhatti + Show 2 more

Open Access

https://doi.org/10.3847/1538-4365/ab3849

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Abstract

Abstract We extract light curves for 4554 objects with 9 < G < 19 in the K2 superstamp observations of the globular cluster M4, including 3784 cluster members, and search for variability. Among cluster-member objects, we detect 66 variables, of which 52 are new discoveries. Among objects not belonging to the cluster, we detect 24 variables, of which 20 are new discoveries. We additionally discover 57 cluster-member suspected variables, 10 cluster-non-member suspected variables, and four variables with ambiguous cluster membership. Our light curves reach sub-millimagnitude precision for the cluster horizontal branch, permitting us to detect asteroseismic activity in six horizontal branch stars outside the instability strip and one inside the strip but with only ∼1 mmag amplitude variability. Nineteen additional stars along the red giant branch also have detected asteroseismic variability. Several eclipsing binaries are found in the cluster, including a 4.6 day detached eclipsing binary and an EW-class eclipsing binary, as well as an EW with uncertain cluster membership and three other candidate EWs. A 22 day detached eclipsing binary is also found outside the cluster. We identify a candidate X-ray binary that is a cluster member with quiescent and periodic ∼20 mmag optical variability. We also obtain high-precision light curves for 10 of the previously known RR Lyrae variables in the cluster and identify one as a candidate Blazhko variable with a Blazhko period in excess of 78 days. We make our light curves publicly available.

Highlights

The globular cluster (GC) M4 (NGC 6121), located in the constellation Scorpius, is the closest GC to Earth at a distance of ∼1.8 kpc (Kaluzny et al 2013b; Braga et al 2015; Neeley et al 2015)
The presentation of the results is organized based on the cluster membership probability of the star, whether it is a horizontal branch (HB) star, and whether a given variability signal is certain, suspected, or indeterminably blended
We adopt the same variability classification scheme, including abbreviations, as used in the General Catalog of Variable Stars (GCVS), March 2017 edition (Samus et al 2017), with additional designations to describe variability not described in this classification scheme

Summary

Introduction

The globular cluster (GC) M4 (NGC 6121), located in the constellation Scorpius, is the closest GC to Earth at a distance of ∼1.8 kpc (Kaluzny et al 2013b; Braga et al 2015; Neeley et al 2015). M4 is rich in variable objects—90 in the current count of Clement et al (2001), June 2016 edition—such as pulsating variables (including dozens of RR Lyrae variables), eclipsing binaries, and cataclysmic variables (Clement et al 2001; Bassa et al 2004; Kaluzny et al 2013a,b; Stetson et al 2014; Samus et al 2017; Watson et al 2017 and references therein). Some recent examples of the scientific utility of these variables include using RR Lyrae variables for an M4 distance determination (e.g., Braga et al 2015) and using M4 eclipsing binaries to provide constraints on the mechanism of formation of close binaries in GCs (Kaluzny et al 2013a). Given the large number of variable objects already known in M4 and the scientific impact of both better understanding known variables and discovering new ones, any data that permits such is of great value

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